Whirling Frenzy's extra attack and Rapid Shot would be a -4 to hit, unless you grab Improved Rapid Shot.
Yeah I know =D
With improved rapid shot and the whirling frenzy variant you have 3 attacks at your bab-2, then whatever iterative attacks you have (each at -2)
I do not see anything (RAW does not make any mention of similar situations that could be used to compare) that would indicate that just because it fires energy it would not benefit from a size increase. (A bigger energy blast would be more damaging, no?)
You do make good points though, a splitting longbow under the effects you mentioned would be shooting 6d6+2x str bonus+2x enhancement bonus+2x arrow enhancement modifiers (i think? not sure about how it affects enchanted arrows, i don't have my champions of ruin book with me) per attack. Pretty solid either way. Certainly enough to keep up with all but the most optimized melee combatants.
If you can pick up weapon focus, specialization, and ranged weapon mastery that would be a solid +3/hit and +4dmg (+8dmg after splitting), and it only requires 4 fighter levels and +8 bab. If you can get access to it (aka you have an extra feat) and intend to go the oversized bow route, a composite greatbow starts doing significantly more damage per shot than a longbow if you have multiple size increases. (With 2 size increases, it is effectively a free +6 damage per split attack, quite possibly worth a feat).